Spark plug



Mayzs, 1929.

H. SCHLAICH SPARK PLUG Filed April 30, 1927 INVENTOR ATTORNEY Patented May V28, 1,929.- j

UNITED STATES PATENT oF-FICE.

application mea april so, 1927. serial no. 197,979.

This invention relates to spark plugs for internal combustion engines and t e lnvention has for its principal object the provislon of a spark plug having a variable spark'gap so constructed as to provide for the productlon of an intense ignition spark 'under all operating conditions.

In sparkplugs as now 1n general usea spark gap of fixed width 1s provlded within the .motor cylinder across which the spark 1s caused to jump. This ga has necessarily been limited tol a width which the spark can readily jump when'conditions are in ost unfavorable to its passage. Such condltions occur when the motor is cold and particularly at starting. If the ignition potential is furnished by a magneto such potential will be at its minimum value when starting the motor firing'points so as to bridge the gap and predue tothe slow rotation thereof, while if the ignition current is furnished by the storage battery which furnishesthe power. for the electric starter, the starter draws on the battery so heavily as to momentarily greatly reduce its volta thereby reducing the igni- 0e `tion potentiaI ,proportionately Furthermore, the resistance of the spark gap is greater in a coldmotor owing to the absence of hot gases and to the incomplete vaporization of the fuel. All ofthese conditions unite to require the use of a comparatively narrow spark gap, if the motor is to start properly.V

On the other hand, it has been found that as the motor warms up, a substantially wider gap produces better ignition. This is partly due to the fact that as wide a gap as it is possible for the spark to jump with regularity is in all cases desirable, and the increased p0- tential available under running conditions permits the use of such wider gap and because it has been found that as the motor warms up the intensity ofthe spark across a gap of fixed width noticeabl diminishes due to diminished resistance of t e gap, caused in part by the rise in temperature of the gases in the gap, and the increased temperature of V the electrodes If a narrow spark gap is used there is moreover danger of short clrcuiting, due to fouling of the plug which may result froma particle of carbon becoming lodged between the ventfthe production of a proper spark.

As a result ofthe conditions named, the

spark gaps used in the ordinary fixed point' .spark plugs constitute a compromise between which would be too to-increase to its maximum width below temperature at which the lvehicle motor northe return o f the the narrow gap most desirable for starting and .the wider gap most desirable for norl mal running. The ap is usually neither narrow enough to ma e the motor start easily, nor wide enough to produce the best ignition when running. In accordance with the present invention a variable spark gap is rovided which is controlled automatically y temperature conditions within the cylinder, so that it is narrow at starting when the motor is cold, and iswidened when the motor heats up, to a width with a coldmotor, but which gives the best results under operating conditions. At the same time 'the present invention lprovides a construction 'for accomplishing. this result which is mechanically simple and eilicient, compact, and of such nature as to provide for the proper conduction away of the heat from the elements of the device so that none of such elements tendstobecome overheated to a point where preignition may occur.

great for easy startingA A further feature of the invention relates to the distortion characteristic of the thermostatic element employed'itv being. a feature that a combination of metallic strips for the thermostatic element i's'contrived such that distortion of the 'element occurs up to a predetermined point, below the normal maximum operating tem erature, and that, due to a change 1n there ative co-eicients of expansion, there. is no further distortion of i the element abovesuch point. Thus the thermostatlc element causes the spark plug gap the .intensifiers Other objects and advantages of the invention will appear from the fo lowing description of a preferred embodiment thereof.

In the drawings forming a part of Ithis I maximum width of the specification in which such' preferred embodiment is illustrated:

Figure 1 is an elevation of a spark plug embodying the improvements of the present invention;

Figure 2 is a bottom plan view of the spark plug shown in Figure 1;

Figure 3 is a fragmentary vertical sectional view of the lower part of the plug;

Figure 3a is a fragmentary elevation showing the same partsof the spark plug shown in Figure 3;

Figure 4 is a plan view of a thickness gauge preferably forming a part of the plug;

Figure 5 is a fragmentary, vertical section of the lower end of a modified embodiment of the spark plug;

Figure 6 is a bottom plan view of the spark plu-g of Figure 5; and

Figure 7 is a fragmentary, enlarged, sectional view illustrating a detail of the connection of the bi-metallic element of Figures 5 and 6 to the spark plug shell.

The present invention is applicable to various constructions of plugs, the particular plug illustrated comprising the conventional eX- ternally threaded metallic shell 1, for securement in the cylinder casting, an insulator 2 of orcelain or other suitable material, a nut 3 c amping the insulator in the shell 1, and a Xed, central electrode 4 carried by the porcelain. Provision is made of a terminal binding nut 5 at the outer' end of the spark plug for securing a wire of the ignition system in conductive relation with the central electrode 4:.

The portion of the insulator whichA is eX- posed inside of the cylinder is in the form of a tapered nose 10, and is surrounded by the skirt 11 of the shell 1, so as to leave a space between the skirt and the nose of the insulator. The provision of this space and its proper shape and proportioning are important for well understood reasons, as upon the proper construction of these parts depends prevention of the accumulation of carbon and oil on the nose of the insulator, which would result in fouling the plug and the establishment of a conducting path between the center electrode 4 and the skirt 11 along the surface of the porcelain. The particular construction of these parts constitutes no part of the present invention, except in so far asthe invention provides for incorporation of the variable gap, to be hereinafter described, in such a way as not to reduce the efliciency of these parts of the plug in the performance of their proper functions.

In the construction illustrated, the means forming the second and movable electrode of the spark plug comprises a bi-metallic strip 6 mounted on the lower end of the skirt 11 of the spark plug shell 1 and carrying the firing point 8, the space between the tip ofu which and the end of the central electrode 4 constitutes the spark gap 13. The end of the strip 6, remote from the firing point 8, is rmly attached to the spark plug shell in any suitable manner. In the particular construction illustrated, the stri is formed with an ear 7 which is set in a mi led recess in the lower face of the skirt 11, said ear being secured in such recess in any suitable manner, such as by crimping or compressing the metal of the skirt tightly against the ear by means of a blow from a punch or by press operation. For the purpose of assuring a permanent mounting which cannot become loose in operation, the ear 7 is preferably provided with a groove 7 a in one of its faces, into which the metal of the shell is deformed by pressure to prevent interlocking. Such groove 7 a is desirably straight and of sufficient length to assure stability of the bi-metallic strip.

The firing point 8 may be formed in any suitable manner, preferably by bending the end of the loi-metallic strip 'radially inward toward the central electrode, as shown, and tapering it down to an appropriate shape. The strip 6 is so formed that the free portion of it, between the point of its attachment to the spark plug shell and the firing point, will warp or become deformed upon a change of temperature so as tomove the firing point 8 toward and from the fixed firing point 4. Preferablythe strip 6 is formed of two layers of metal firmly united together, the inner layer of which has a greater co-efficient of expansion than the outer layer, so that when the strip becomes heated it will warp in a direction to widen the spark gap.

In the embodiment of the invention illustrated, it will be seen that the free portion of .the strip 6 extends parallel to the plane of the lower end of the skirt 11, that it in general follows the cylindrical curvature of the skirt so as to over-lie the lower end thereof, and that one face of it is brought into close proximity to, or preferably into substantial contact with the end of the skirt, such Contact preferably extending throughout all or the larger part of the length of the strip. This construction embodies and illustrates very important features of my invention.

In the first place it is very important to have the movable firing point and its thermoresponsive supporting means so constructed and arranged as not to become heated during the operation of the engine to such a degree as to cause preignition. In installations where the tendency to overheat is pronounced, overheating may be prevented by the extended contact between the strip and the end of the spark plug shell. It will be understood that the spark plug shell, being screwed directly into the cylinder casting is Imaintained at a suitable temperature comparable to that of the castingitself which latter temperature is controlled by the water jacket or other cooling means of the engine. The bi-metallic strip being mounted in close contact with the end of the spark plug shell, which contact, as stated, preferably extends substantially throughout the length of the strip, readily transfers its heat to the end of the shell, and is thus sufficiently cooled to prevent preignition-although not to such an extent as to prevent it from warping to open the spark gap. The contact between the edge of the strip and the end of the shell should bev made as extensive and as close as is convenient. The strip may be brought into such close contact with the end of the shell that there isactually some pressure between them, just so long as the engagement is not sutlicient to prevent the proper change in shape of the strip at different temperatures. The force available for movement of the strip, due to relative expansion of the metals therein, is very considerable and can overcome considerable friction between the vedge of the strip and the end of the shell.

For most efficient heat dissipation there should not be enough space between the edge of the strip and the end of the shell to permit any free circulation of the gases between the' two at the points where it is expected that the heat shall be transferred because such gases would act as a thermal insulator and would cut down the heat transference.

While it is desirable, as stated, that the bimetallic strip` 6 be mounted for movement parallel to the lower face of the shell and in actual contact therewith, precision of adjustment to secure closeor continuous contact is not in fact imperative. Where the bimetallic strip does not closely engage the shell, the accumulation of carbon on the lower face of the shell will establish a heat transferring relationship and assist in maintaining the, temperature of the bi-metallic strip, including the firing point 8, within the desired temperature range. This carbon deposit will form and transfer heat provided the space is not so great as to permit free circulation of gases. Such circulation would prevent the formation of such a carbon deposit. The depositing of carbon on the shell will not, however, result in building up a thick carbon layer between the shell and the ybi-metallic strip or around the strip, for the reason that -the bi-metallic strip will have wiping or scraping contact with the carbon and will prevent an accumulation of the carbon beyond the thickness which may actually be required for bridging the very short space b between the bi-metallic strip and the shell.

The arrangement of the strip 6 parallel to the plane of the end of the plug and with its free part overlying the poses the strip in such manner that it does not choke or interfere with the action of the space 12 between the skirt' and the nose of the porcelain. It therefore hasno tendency to sion up to the point at which it is desired to end of the skirt, discause fouling or short circuiting of the plug. The location of the strip in the plane parallel to that in the end of the plug also results in a very compact constructionin which there are no extended parts which might reduce the cylinder clearances. The concentric arrangement of the strip 6 with reference to the end of the plug also permits the free portion of the strip to be extended to the necessary length to produce the desired amount of movement under temperature change, while still maintaining the. compact form. By attaching one end of the strip 6 directly to the plug shell as by means of the ear 7 fitting in the slot, and by forming the firing point 8 as an integral part of the strip, all separate elements, such as pins, screws and the like which might become loose or detached are avoided.

Itis further to be observed that in my construction the firing point having the thermoresponsive mounting is the point which is carried by the shell of the plug rather than vthe. central electrode. This is important for two reasons: first, that it provides for the transference of the heat of the thermo-responsive element directly to the spark plug shell, and so to the cylinder casting, and cooling system,tl1us preventing such overheating of the thermo-responsive element as would result in preignition; and second, that it avoids the necessity of associating any moving or a'djustable parts with the porcelain. 1f it were attempted. to associate such moving parts with the porcelain, diiiicultieswould be encountered in preventing breakage of the porcelain or other insulating material due to such movement.

As has been stated, it is a further object of the invention to so construct the thermostatic element that carries the movable electrode that the gap will be automatically increased to its maximum width somewhat below the normal operating temperature, and will re-f main at such width without the necessity vof 1160 employing an extraneous stop so that further heating ofthe element does not subject the parts thereof to strain nor result in increasing the spark gap to a length greater than that desired.

In accordance with the presentinvention, this is accomplished by combining into a bimetallic strip two metals or alloys which have substantially different co-efiicients of expanarrest distortion of the thermostatic element, ut whose co-etficients of expansion become and remain substantially equal at and above such predetermined temperature.

,It has been found that this result can be obtained in a very satisfactory manner by making the outer metallic strip of the thermstatic element of invar which is a nickel steel alloy, having approximately thirty-six per cent (-36%) nickel andsixty-four per cent 130 i (64%) steel and the inner metallic strip of the thermostatic element of an alloy having substantially the following composition:

Carbon .50 Manganese .70 Silicon .75 Chromium 8.00 Nickel 22.00 Copper 2.00 Cobalt 1.00 Iron 64.05

As is well known, invar has an almost negligible co-eflicient of expansion from 0 C. to approximately 100 C., but the co-eiiicient of expansion increases rapidly from 100o C.

to approximately 300 C. Above the latter temperature the co-eficient of expansion of invar becomes substantially uniform but is quite substantial, being approximately equal to the co-eicient of expansion of the alloy of which the other strip of the thermostatic element is constructed. Said other strip has a substantially uniform co-efficient of expansion both below and above 300 C., and in fact throughout substantially the entire operating range of temperatures to which it is subjected in use.

With the thermostatic element constructed as described, it will be apparent that the thermostatic element will attain is maximum distortion at the temperature at which the coefficients of expansion of the two metallic strips become substantially equal; that is to say, at about 300 C.

With the construction as shown and described herein, the minimum spark plug gap is adjusted to fifteenthousandths of an inch, and, upon heating, the thermostatic element automatically increases the gap to approxi mately thirty-four thousandths of an inch and then causes the gap to maintain this width irrespective of further heating until the' thermostatic element is again cooled below the temperature at which such maximum length of the gap is ordinarily reached.

For the purpose of providing a thickness gauge to enable the ap to be measured when the parts are colepnovision is made of a washer 9 which is normally held in place upon the upper end of the spark plug by the terminal binding nut 5. When it is desired to test the width of the gap the washer 9 may be removed from the spar: plug and an ear l14: on the washer, of predetermined thickness,

may be used as a thickness gauge. If desired the thickness of this ear may be marked directly thereon.

The spark plug illustrated in Fi ures 5 to 7 is the same as the spark plug a ready described, with the exception that the means for attaching the bi-metallic strip to the shell is modified.

In this form of the invention the bi-metallic strip 6 is provided with pins 15 which may be 4effects a very firm securement of the bimetallicstrip to the shell. For the purpose of further strengthening the connection, a punch or press may be subsequently applied to exert pressure transversely of the pins through the lower end of the shell at that portion which receives the pins. The area of application of such pressure is indicated, for example, by the depressed portion 17 of Fig ure 5.

While I have illustrated and described in detail a preferred form of my invention, it is to be understood that changes may be made therein and the invention embodied in other structures. I do not, therefore, desire to limit myself to the specific construction illusrated, but intend to cover my invention broadly in whatever form its principle may be utilized.

I claim:

1. In a spark plug, in combination, a metallic shell adapted to be located in intimate, heat transferring relation with the cooling system of the motor, a fixed central electrode insulated from said shell, a movable electrode cooperating with the fixed electrode, and a thermostaticelement controlling the movable electrode, said thermostatic element being mounted on the shell and being disposed throughout substantially its entire length in efficient heat transferring relation to the shell.

2. In a spark plug, in combination, a metallic shell adapted to be located in intimate, heat transferring relation with the cooling system of the motor, a fixed central electrode insulated from said shell, a movable electrode cooperating with the fixed electrode, and a therm'ostatic element controllin the movable electrode, said thermostatic e ement being mounted on said shell and projecting below the lower face thereof, and being arranged to make wiping contact throughout substantially its entire length with the lower face of the shell.

3. In a spark plug, in combination, a metallic shell ada ted to be located in intimate, heat transferring relation with the cooling system of the motor, a fixed central electrode insulated from said shell, a movable electrode cooperatin with the fixed electrode, and a thermostatic element controlling the movable electrode, said element being mounted on the shell and movable substantiall parallel to the lower face of the shell an 1n proximity throughout substantially its entire length, to said lower face of the shell.

4. In a spark plug, in combination, a metallic shell adapted to be located in intimate,

heat transferring relation with the cooling p system of the motor, a fixed central electrode insulated from said shell, a movable electrode cooperating with the fixed electrode, and a bi-metallic strip integral with the movableelectrode and intimately united with the shell, said element bein movable parallel to the lower face of the s ell and in close proximity thereto. y,

5. In a spark plug, the combination of a shell, an insulator, an electrode mounted in said insulator and a bi-metallic stripattached at one end to the end ofsai/d shell and carrying a movable firing point, said bi-metallic strip moving in a plane parallel to the plane of the end of said shell. 6. In a spark plug, in combination, a metallic shell adapted to be located in intimate heat transferring relation with the cooling system 'of the motor, a fixed central electrode insulated from said shell, a movable electrode cooperating with the fixed electrode, and a bimetallic element mounted on said shell and extending parallel to the `lower face of the shell, sa1d element lJoeing distortable by temperature changes aard arran ed when so distorted to produce movement o the movable electrode in a direction substantially parallel to the lower face of the shell.

7. In a spark plug, in combination, a metallic shell adapted to be located in intimate heat transferring relation with the cooling system of the motor, a fixed central electrode insulated from said shell, a movable electrode c0- operatin with the fixed electrode, and an arcuate, i-metallic element carrying the movable electrode, said bi-metallic element being mountedon the shell and substantially con,- centrically disposed with relation thereto.

8. In a spark plug, the combination with a central, fixed sparklng electrode, of an insuy late r carrying said electrode, a metallic shell havlng a skirt portion so spaced from the insulator that an insulating and carbon scavengingspace is provided between the .skirt and the insulator, said skirt portion of the metallic shell being threaded for securement in intimate, heat transferring relationship to the cooling system of a motor, a second .movable electrode cooperative with the fixed electrode to form a sparking gap, and a bi-metallic strip mounted in the lower end of the shell and carrying the movable electrode, said bi-metallic strip beingmounted edgewise upon the lower face of the skirt r. portion so that obstruction of the scavenging space by it is substantially avoided.

9. In a spark plug, in combination, a metallic shell adapted to be located v in intimate, heat transferring relation with the cooling system of the motor, a fixed central electrode insulated from said shell, a movable electrode cooperating with the fixed electrode, and a bimetallic strip mounted on the shell and carrying the movable electrode, said bi-metallic strip being provided withb an ear set in to a.v recess in the shell, and said ear being provlded with a groove into which the metal of the shell n is pressed.

10. The method of making aspark plug, comprising a metallic shell and an electrode carrying bi-metallic strip mounted thereon, which comprises providing a shell having a recess in the lower end thereof, providing a bi-metallic strip having an ear adapted to be inserted in said recess, which ear is provided with a depression in one face thereof, inserting the ear in the recess of the shell and compressing the shell transversely of said ear to press the metal of the shell into said recess.

11. In a spark plug, the combination with a fixed sparking electrode, of a second sparking electrode movable to change the length of the spark gap, 'and a thermostatic element controlling the movement of said second electrode in accordance, with temperature conditions, comprising two metallic strips having different co-efiicints-of expansion up to a certain temperature, and substantially equal co-efiicients of expansion above such temperature.

12. In a spark plug, the combination with a fixed sparking electrode, of a second sparking electrode movable to change the length of the spark gap, and a thermostat-ic element controlling the movement of said second electrode in accordance with temperature. conditions, said thermostatic element being characterized by the fact that it is distortable by temperature changes up to a certain temperature, and is inherently substantially non-distortable above such temperature.

13. In a spark plug the combination with a fixed electrode of a second electrode composed of two metallic strips having different coefficients of expansion up to a certain temperature, and substantially equal co-efiicients` of expansion above such temperature.

14. In a spark plug the combination with a fixed electrode of a second electrode composed of two metallic strips having different co-efiicients of expansion throughout a predetermined temperature range, and substantially equal co-efiicients of expansion throughout another predetermined temperature range.

15. In a spark plug, a shell adapted for se- -curement to a motor cylinder, axed sparking ing terminal, an arcuate, loi-metallic strip secured at one end to a marginal portion of the spark plug and extending edgewise around the central sparking terminal in a 5 plane substantially normal to the axis of the spark plug, and a second sparking terminal carried at the free end of said strip, said strip being constructed to expand as the temperature rises, and so tocarrylthe terminal mounted on it away from the fixed terminal. 10

In testimony Whereofllave ailixed my signature to this specification.

HERMAN SCHLAIGH. 

